This invention relates to optical transmission systems, and in particular to systems in which transmitted signals are wave division multiplexed.
Wave division multiplexing (WDM)is being introduced as a means of increasing the traffic handling capacity of optical fiber transmission systems. In a WDM system each individual fiber carried signals at a number of wavelengths, typically four. When these signals are transmitted over long distances, periodic regeneration of the signals is necessary. Currently, this amplification is generally effected by demultiplexing the different wavelengths and then converting the light signals to corresponding electrical signals. These electrical signals are regenerated and then reconverted to light signals.
In an effort to reduce equipment costs, attempts have been made to replace the optoelectronic regenerators with optical amplifiers, e.g. erbium fiber amplifiers. Such amplifiers have the advantage both of relatively low cost and the ability to amplify all the wavelengths without the need for demultiplexing.
In most single wavelength optically amplified transmission systems, each optical amplifier is arranged to provide a substantially constant total mean power output to ensure that transmission quality is maintained. Attempts to introduce amplifiers of this type to WDM systems have not proved successful. The basic problem is that of determining the appropriate amplification factor of an amplifier from the input signal power. Current signals measure the total input power irrespective of wavelength. If all the multiplexer wavelengths are present at the input of an amplifier then an appropriate amplification factor can be determined. However if one or more wavelength is missing from the input signal, e.g. under fault conditions, this causes a corresponding drop in the total input power. To compensate for this power drop the amplifier increases its gain and thus amplifies the remaining wavelength with an effectively increased gain factor resulting in too high an output power from these wavelengths. For example, temporary loss of three wavelengths in a four-wavelength WDM system can result in a 6 dB increase in the power output of the remaining wavelength. This can incur serious penalties arising from non-linearity induced by spectral broadening. In particular, this spectral broadening results in an increased time dispersion of the signal.
The object of the present invention is to minimise or to overcome this disadvantage.